(1) Field of the Invention
The present invention relates generally to voltage control unit circuits, and more particularly, to DC-DC converters having an analog MOS current sense circuit.
(2) Description of the Prior Art
Current sense circuits are widely used in integrated circuits. If a potentially large output current, or load current, must be driven by an on-chip switch, a current sense circuit may be used to detect the relative or absolute value of this current. The current level may be monitored to prevent damage to the switch, or to the integrated circuit from either a short circuit or a simple overloading.
Referring now to FIG. 1 prior art, a simplified schematic of a current sense circuit, as disclosed in the U.S. Pat. No. 6,479,975 by Plankensteiner et al. is illustrated. The load current is sensed for a low-side driver output comprising an NMOS output transistor 104. The gate of said transistor 104 is coupled to a control signal, CONTROL 116. The drain is coupled to the output pin of the circuit, OUT 142. The source is preferably coupled to ground 128. A load 100 is coupled between OUT 142 and an external voltage source 124. Alternatively, the internal voltage source could be used. The low-side driver 104 is turned ON when the CONTROL voltage is above the threshold of transistor 104.
The current sensing is provided by the sensing transistor 108, constant current source 112, and comparator 130 in combination with the output transistor 104. More specifically, the gate of the sensing transistor 108 is coupled to CONTROL 116. The source is coupled to ground 128. In this configuration, sensing transistor 108 receives the same gate drive as 104.
A constant current source 112 is coupled to the drain of the sensing transistor 108. In this configuration, the ON-state drain current of transistor 108 corresponds exactly to the current generated by said current source 112 since the current flow into the high impedance inputs of the comparator 130 is negligible. Further, as long as current source 112 is below the saturation current of sense transistor 108, the sense transistor will operate in the linear region. By maintaining sense transistor 108 operation in the linear, or ohmic region, the operating characteristics of transistor 108 will match those of output transistor 104.
Several prior art inventions describe current sensing circuits used in DC-DC converters:
U.S. Pat. No. 6,441,597 (to Lethellier) discloses a current sense circuit for a DC-to-DC power converter accurately sensing the output inductor current without adversely affecting efficiency of the power converter. The current sense circuit produces a current sense signal having amplitude sufficiently above the noise floor so that accurate load control of the power converter is achieved. Specifically, the DC-to-DC power converter includes at least one power switch connected to an input voltage source. At least one phase sensing switch is connected to the input voltage source in parallel with the at least one power switch. A pulse width modulation circuit provides common control pulses for the at least one power switch and the at least one phase sensing switch responsive to a current sense signal. An output inductor is connected to the at least one power switch and to a load. A current sensor is coupled to the output inductor and providing the current sense signal to the pulse width modulation circuit corresponding to current passing through an internal DC resistance of the output inductor. The current sensor further includes a filter that includes an on-state resistance of the at least one power switch. The current sensor further includes a second filter adapted to remove noise from the current sense signal when the at least one phase sensing switch and the at least one power switch change state.
U.S. Pat. No. 6,426,612 (to Rozsypal) teaches a current sense circuit which receives a signal indicative of output current flow of an up/down DC-DC converter during up-conversion and down-conversion modes. The current sense circuit provides a logic signal indicative of the rate of change of the current flow for both modes of operation. A comparator receives a selectable voltage reference generated by voltage reference. A first voltage reference is selected during an increasing current flow and a second voltage reference is selected during a decreasing current flow, thereby regulating the output current to a fixed average value. If the length of time during continuous current flow of converter exceeds a predetermined amount of time, a signal is issued and externally processed, which subsequently disables converter by an external signal.